This invention pertains to satellite communication systems and more particularly is concerned with systems correcting for frequency errors.
A typical satellite communication system includes a hub earth station, a geostationary satellite, and a plurality of terminal earth stations. The hub provides a continuous outbound transmission, while the terminals provide bursts of inbound transmissions. Transmission from any earth station to the satellite is called uplink. Transmission from the satellite to earth stations is called downlink.
When the satellite receives an uplink signal, it frequency translates the signal by mixing with a local oscillator to a downlink carrier frequency, and transmits the signal downlink to the earth stations at the downlink carrier frequency. The downlink carrier frequency differs from the uplink carrier frequency by a standard frequency offset, e.g. 2.3 Ghz, and an unknown frequency offset error due to the satellite's local oscillator and doppler shift due to satellite movement. The hub burst demodulators have a narrow frequency, necessitating reduction of the frequency uncertainty of the received signal.
Some satellite communication equipment use a pilot signal to determine frequency shift. This technique adds to the cost and complexity of a system and uses up part of the available frequency spectrum.